This invention relates to a resonant piston compressor that is reciprocated by a linear motor and, in particular, to a reciprocating compressor of this character wherein the multistage cylinder forces acting on both sides of the motor plunger unit are closely balanced and the natural frequency of the plunger unit is tuned to the exciting frequency of the motor to provide resonance at the design point of the system.
Resonant piston compressors have been known and used for some time. These machines usually involve a pair of opposed cylinders having pistons of equal size that are arranged to reciprocate so that one is on the compression side of the stroke while the other is on the suction side. As explained by Curwen in U.S. Pat. Nos. 3,303,990; 3,329,334 and 3,156,405, all of which are assigned to the present assignee, the pistons can be driven by means of coacting solenoids wherein each piston contains an outwardly disposed iron core that is coupled electromagnetically to a transducer. By pulsing the transducers in an ordered sequence, the pistons are made to reciprocate within the compression cylinders. A fairly large flexure spring unit is used to connect the piston assembly of the machine to the machine casing. The spring includes two U-shaped sections that protrude outwardly through the casing and which are tuned so that the mechanical and electromagnetic frequencies of the system are at resonance.
Although the Curwen type compressor works quite well in a number of different applications, the cylinders cannot be interstaged to raise the pressure of the working substance in steps and the output capacity of the compressor is thus limited. As can be seen, if the opposed cylinders of the Curwen compressor were staged, the forces acting on the piston assembly would become unbalanced and the flexure spring would be unable to hold the system at or near resonance. It should also be noted that the flexure spring takes up a good deal of space to either side of the casing. Furthermore, because the flexure spring is not internally contained within the casing, it is extremely difficult to immerse the compressor in a coolant and other, less desirable, forms of cooling must be employed to dissipate the heat of compression.
In U.S. Pat. Nos. 3,937,600; 4,353,220 and 4,067,667, all which are assigned to the present assignee, there is shown another type of resonant piston compressor wherein the flexure spring is replaced by a gas spring system for attaining resonance. The gas spring is fully contained within the machine casing and the entire assembly is housed in a fluid tight shell immersed in a bath of cooling oil. Here again two symmetrical pistons are driven in opposition by a linear motor to balance the forces on either side of the rotor. Dashpots, which act as gas springs, are operatively connected to each piston. The stiffness of each gas spring is adjusted in reference to the oscillation of the motor armature assembly to produce resonance at the desired design point. Upsetting the pressure balance between cylinders, as for example by staging the compression, would again adversely affect the resonant characteristics of the system.